October 2, 2011

Harmonic Oscillators

Buchla made the 148 Harmonic Generator in 1969. It is a sawtooth oscillator core, the same as the 158, with waveshapers creating 9 harmonics above the fundamental. The even harmonics are created with full wave rectifiers doubling lower frequency triangles. The odd harmonics are shaped using a series of diode clippers that mix the triangle with a clipped version to make a higher frequency triangle. The effect is a wavefolder, the precursor to the Timbre circuit in the 259.

I built a modified version of that design 10 years ago. The original has no -15volt rail, so it AC couples the signal in many places. I eliminated all these caps and powered the circuit from +/-15 volts. I never got the quality of the waveforms to where I wanted them. I have since worked on a Buchla 148 and found the waveshapes to be about the same as my clone. They sound a little wavetabley, especially the higher odd harmonics.

Legend tells that Buchla made a version of the Harmonic Generator for the 200 series. It was based on the design of the 148, but had a few extra parts added. It had reversing attenuators on the CV ins and an output mixer with sliders and even/odd outs. I have never seen this module and don't even know the model number.

In 2009 I started designing my take on this forgotten concept. I made up my own waveshapers using distorted CMOS opamps, like the 259 Timbre circuit, instead of diode clippers. This does not sound good. I have since thrown the design out.

Finally, I have my finalized design. The Harmonic Oscillator. The analog oscillator core puts out saw, square, triangle, and spike (208 style). The waveshaper puts out 10 decent quality sine waves. The voltage controlled mixer has sliders and CV ins for all channels as well as scanning circuitry (sort of freq. and bandwidth for the harmonics) and tilt (to favor the high harmonics or the low harmonics from voltage).

March 18, 2011

Prototype Music Easel!?

The Music Easel is the street name for the combination of the Buchla 208 and 218 touchplate keyboard. I was recently called upon to repair the prototype unit, built in 1973. This unit was resurrected in 1987 and had since been in a basement, used from time to time without ever being moved. He built a very cool blue wood enclosure for it with a built in linear power supply.

When it changed hands, several problems appeared. The 208 module is built from a large motherboard with 12 perpendicular daughter cards, attached with .156" Molex connectors. The biggest problem was that the connectors had oxidized and the cards had to be jiggled around to get several of them to work. The 20 years that this thing sat in the same spot it was fine, but I guess shipping did a number on them. I replaced the female side on all 12 cards and things started to get better.

The Oscillator and some other parts were still not working properly, so I set out to find the problems. Being the prototype, there are several things that are not the same as other units. The panel has a sine wave in the modulation oscillator, when it actually puts out a triangle, the memory card edgecard connector is smaller, the Envelope and Pulser sliders are labeled backwards and the modulation oscillator's banana output jack covers some text. The boards are filled with cut traces and flying resistors. It was a lot of fun.

Finally, I got to the 218. It was only triggering notes while you touched one of the grounded frets. The original owner thought this was normal, but I have used these before so I had to figure out what was wrong. I found that the resistors controlling the keyboard signal's gain were different on the schematic, the parts overlay AND the picture that I had taken of a working unit when I serviced it. I tweaked these values until the keys triggered normally.

Good Ol' Filters

The Lopass Gate is widely recognized as a sweet sounding filter with a Vactrol-induced slew to it. Set into the top switch position, it is a Sallen-Key filter very similar to the Korg MS-10/MS-20 filter. The LED/photocell elements, known as Vactrols, add something around 10ms of slew on the attack and 100ms of slew on the decay. This is more than an OTA filter for sure, but what if there was some REAL slew on the CV?

Then, there's the Buchla 192. A simple, nonresonant, 2 pole lopass filter, featuring an all discrete transistor signal path with the control elements made from photocells and incandescent lamps in a little bit of heat shrink tubing! The original run (it's quite rare, but I don't know how many are out there) of this module didn't even have a CV input. It has a slew of several seconds when the cutoff is changed.

The one that landed on my workbench was living behind a Dual Reverb front panel with the labels scratched off. It was not functioning. Since I discovered at least one dead "optical element" and this device was such a hack anyway, the owner and I decided to mod it for actual VTL5C3/2 Vactrols. After all was said and done, it is a very interesting sounding lopass. Sort of like a 292 in lopass mode, but with an all discrete signal path.